Loading page content...
Calculate exact ages, date spans, business days, and time — down to the second
Age and date calculations feel simple until you need exact precision — leap years, variable month lengths, business-day exclusions, time zone offsets, and the subtle difference between ISO dates and locale-specific formats all create pitfalls that trip up manual counting. Whether you need your exact age for a passport application, the precise business days between contract signing and closing, or decimal hours for a payroll timesheet, this guide covers every scenario. Our calculators follow NIST timekeeping standards and handle dates from Year 1 to Year 9999, including all Gregorian leap year edge cases. Try the age calculator, date calculator, or hours calculator for instant, precise results.
Calculating exact age means counting completed units of time from birthdate to today. The algorithm: (1) compute raw year difference, (2) check if today’s month-day is before the birth month-day — if so, subtract one year and note the remaining months/days, (3) count remaining months, (4) count remaining days. Key edge cases: Feb 29 birthdays — in non-leap years, the birthday is treated as Feb 28 (UK) or Mar 1 (some US states). Cross-century dates: 2000 is a leap year (divisible by 400), 2100 will NOT be (divisible by 100 but not 400). For legal documents, passport applications, and age verification, always use the official birth certificate date. Our age calculator computes age to the exact second and handles all Gregorian edge cases including people born on Feb 29 and December 31.
The most reliable method for counting days between two dates is the Julian Day Number (JDN) approach, which assigns a serial integer to every calendar day. JDN for any Gregorian date (Y, M, D): JDN = 367×Y − ⌊7×(Y + ⌊(M+9)/12⌋)/4⌋ + ⌊275×M/9⌋ + D + 1721013. Days between two dates = JDN2 − JDN1. Example: Jan 1, 2000 to May 2, 2026 = 9,618 days. This automatically handles all 9 leap years in that range. Quick methods: multiply year span by 365.2425 for estimates; in Excel/Google Sheets, subtract two date cells directly (=B1−A1). Use our date calculator for instant calendar-day and business-day results.
Business days (working days) exclude weekends and public holidays — the relevant unit for contract deadlines, loan processing, shipping estimates, and payroll cycles. US Federal Holidays 2026 (11 total): New Year’s Day (Jan 1), MLK Day (Jan 19), Presidents’ Day (Feb 16), Memorial Day (May 25), Juneteenth (Jun 19), Independence Day (Jul 3, observed), Labor Day (Sep 7), Columbus Day (Oct 12), Veterans Day (Nov 11), Thanksgiving (Nov 26), Christmas Day (Dec 25). UK Bank Holidays 2026 (8): New Year’s Day, Good Friday, Easter Monday, Early May Bank Holiday, Spring Bank Holiday, Summer Bank Holiday, Christmas Day, Boxing Day. Business days formula: total calendar days − Saturdays − Sundays − holidays on weekdays. Pro tip: always specify whether a contract clause means calendar days or business days — a 30-business-day window can span 42+ calendar days when holidays cluster.
Date arithmetic has a well-known pitfall: end-of-month clamping. When adding months, the result must be a valid calendar date: Jan 31 + 1 month → Feb 28 (2025) or Feb 29 (2024) — never March 3. Oct 31 + 3 months → Jan 31 (exact, since Jan has 31 days). Mar 31 − 1 month → Feb 28/29 (clamped). Adding days is always exact: advance the day count, rolling over months/years as needed. Use days (not months) for 30-day, 60-day, 90-day contractual windows to avoid ambiguity. Year addition: adding 1 year to Feb 29 (leap) → Feb 28 (non-leap). Important for annual subscription renewals, warranty expiry, and loan maturity dates. Our date calculator supports adding any combination of years + months + weeks + days, showing the result date with day-of-week.
The Doomsday Algorithm (John Conway, 1973) lets you mentally calculate the day of the week for any historical or future date. It relies on anchor dates (Doomsdays) that always share the same weekday within a year. The pairs 4/4, 6/6, 8/8, 10/10, 12/12, 5/9, 9/5, 7/11, 11/7 plus the last day of February all fall on the same weekday. 2026 Doomsday = Wednesday. So April 4, June 6, August 8, October 10, December 12 are all Wednesdays. To find May 2, 2026: April 4 is a Doomsday (Wed). May 2 = April 4 + 28 days = 4 exact weeks → Wednesday. Century anchors: 1800s = Friday, 1900s = Wednesday, 2000s = Tuesday, 2100s = Sunday. This reduces any date to a 3-step mental calculation. Use our date calculator for instant day-of-week lookup.
The world has 400+ named time zones grouped around UTC (Coordinated Universal Time) offsets from UTC−12:00 to UTC+14:00. Non-integer offsets: India = UTC+5:30, Nepal = UTC+5:45, Iran = UTC+3:30, Newfoundland = UTC−3:30, Chatham Islands NZ = UTC+12:45. DST (Daylight Saving Time): ~70 countries shift +1h in spring, −1h in fall. US springs forward 2nd Sunday of March, falls back 1st Sunday of November. No DST: China, India, Japan, most of Africa. Common UTC conversions: New York EST = UTC−5 (EDT = UTC−4); London GMT = UTC+0 (BST = UTC+1); Paris/Berlin CET = UTC+1 (CEST = UTC+2); Mumbai IST = UTC+5:30; Tokyo JST = UTC+9; Sydney AEST = UTC+10 (AEDT = UTC+11). Best practice: store timestamps in UTC, convert to local time on display only. Use ISO 8601 with explicit offset: 2026-05-02T14:30:00-05:00.
For payroll and billing, time must be in decimal hours: hours + (minutes ÷ 60). Quick reference: 0:15 = 0.25h; 0:30 = 0.50h; 0:45 = 0.75h; 7:30 = 7.50h; 8:45 = 8.75h. Overnight shifts: if end time is earlier than start, add 24h to end. Example: 10 PM to 6 AM = 6:00 + 24:00 − 22:00 = 8.00 hours. FLSA overtime rules (US): per the Fair Labor Standards Act, employees must receive 1.5× regular rate for hours exceeding 40 per workweek. California and Alaska also have daily overtime for hours over 8/day. Payroll week: employers define their own workweek (any 7-day period); it need not start on Monday. Our hours calculator computes regular hours, overtime hours, and gross pay for any hourly rate and multiplier.
The Unix timestamp represents time as seconds elapsed since 1970-01-01 00:00:00 UTC. It is universal across operating systems and programming languages. Notable timestamps: Y2K = 946,684,800; May 2, 2026 = 1,746,057,600; Year 2038 problem = 2,147,483,647 (Jan 19, 2038 03:14:07 UTC, 32-bit integer overflow). Modern 64-bit systems won’t overflow until year 292,277,026,596. ISO 8601 formats: full datetime = 2026-05-02T14:30:00Z; with offset = 2026-05-02T09:30:00-05:00; date only = 2026-05-02; week date = 2026-W18-5 (Friday of week 18); ordinal = 2026-122 (122nd day of 2026). Always prefer ISO 8601 in APIs, databases, and spreadsheets to eliminate MM/DD/YYYY vs DD/MM/YYYY ambiguity. JavaScript: new Date().toISOString() returns ISO 8601 UTC. Python: datetime.now(timezone.utc).isoformat().
The Gregorian calendar (Pope Gregory XIII, 1582) corrected the Julian calendar’s 0.0078-day/year drift by refining the leap year rule. The Julian calendar added a leap year every 4 years, accumulating 1 extra day per 128 years. The Gregorian fix: century years (divisible by 100) are not leap years unless also divisible by 400. The 1582 reform skipped 10 days: Thursday October 4 → Friday October 15 in Catholic countries. Protestant countries adopted Gregorian later — Britain and colonies in 1752 (losing 11 days), Russia in 1918 (losing 13 days). Complete leap year rules: (1) Year divisible by 4 → leap year (e.g., 2024, 2028). (2) Year divisible by 100 → NOT a leap year (1900, 2100). (3) Year divisible by 400 → IS a leap year (1600, 2000). In a 400-year Gregorian cycle there are exactly 97 leap years, giving a mean year of 365.2425 days. NIST defines UTC via atomic clocks, occasionally adding leap seconds to keep UTC within 0.9 seconds of Earth’s rotation (UT1).
Countdown calculations measure time remaining until a future event. Steps: (1) get current datetime in UTC, (2) get target datetime in UTC using ISO 8601, (3) subtract to get total seconds, (4) decompose: ÷3600 = hours; remainder ÷60 = minutes; remainder = seconds; then hours÷24 = days; days÷7 = weeks. Popular countdowns from May 2, 2026: New Year 2027 = 244 days; Thanksgiving 2026 (Nov 26) = 208 days; Summer Solstice 2026 (Jun 21) = 50 days; US Election Day 2028 ≈ 918 days. Anniversary calculation: add N years to the start date with leap-year clamping (Feb 29 → Feb 28 in non-leap years). Critical for wedding anniversaries, work anniversaries, domain renewal dates, and subscription expiry tracking. Our date calculator includes countdown mode and anniversary finder showing day-of-week for any future date.
13 free tools linked to this guide
Calculate your exact age in years, months, days, hours, minutes, and seconds from any birthdate.
calculator →Find the difference between any two dates, add/subtract days/months, and count business days.
calculator →Calculate hours worked, convert time to decimal for payroll, and compute overtime pay.
calculator →Convert times between any two world time zones with automatic DST adjustment.
calculator →Countdown to any future date in days, weeks, and months.
calculator →Count business days between dates excluding weekends and US/UK/CA public holidays.
calculator →Convert Unix timestamps to human-readable dates and vice versa.
calculator →Find the ISO 8601 week number for any date and all dates within a given week.
calculator →Science-based sleep duration targets, sleep cycle math, and recovery optimization.
guide →Understand gross vs net pay, FICA deductions, and FLSA overtime calculation rules.
guide →Convert length, weight, temperature, volume, speed, and digital units with formulas.
guide →Calculate retirement age targets, contribution timelines, and Social Security benefit dates.
guide →Due date calculator, gestational age by weeks, and trimester date milestones.
guide →Browse our complete tools calculator suite — free, instant, and trusted by professionals worldwide.